Apolipoprotein A-V deficiency enhances chylomicron production in lymph fistula mice

Abstract

Apolipoprotein A-V (apoA-V), a liver-synthesized apolipoprotein discovered in 2001, strongly modulates fasting plasma triglycerides (TG). Little is reported on the effect of apoA-V on postprandial plasma TG, an independent predictor for atherosclerosis. Overexpressing apoA-V in mice suppresses postprandial TG, but mechanisms focus on increased lipolysis or clearance of remnant particles. Unknown is whether apoA-V suppresses the absorption of dietary lipids by the gut. This study examines how apoA-V deficiency affects the steady-state absorption and lymphatic transport of dietary lipids in chow-fed mice. Using apoA-V knockout (KO, n = 8) and wild-type (WT, n = 8) lymph fistula mice, we analyzed the uptake and lymphatic transport of lipids during a continuous infusion of an emulsion containing [(3)H]triolein and [(14)C]cholesterol. ApoA-V KO mice showed a twofold increase in (3)H (P < 0.001) and a threefold increase in (14)C (P < 0.001) transport into the lymph compared with WT. The increased lymphatic transport was accompanied by a twofold reduction (P < 0.05) in mucosal (3)H, suggesting that apoA-V KO mice more rapidly secreted [(3)H]TG out of the mucosa into the lymph. ApoA-V KO mice also produced chylomicrons more rapidly than WT (P < 0.05), as measured by the transit time of [(14)C]oleic acid from the intestinal lumen to lymph. Interestingly, apoA-V KO mice produced a steadily increasing number of chylomicron particles over time, as measured by lymphatic apoB output. The data suggest that apoA-V suppresses the production of chylomicrons, playing a previously unknown role in lipid metabolism that may contribute to the postprandial hypertriglyceridemia associated with apoA-V deficiency.

Keywords: absorption; intestine.

Fig. 1.

Lymph flow was maintained at 0.2–0.3 ml/h in apolipoprotein A-V (apoA-V) knockout (KO) and wild-type (WT) mice. Mesenteric lymph flow was measured hourly for 6 h. *P < 0.05. Values are means ± SE.

Fig. 2.

ApoA-V KO mice show increased triglyceride (TG) and cholesterol transport into the lymph compared with WT. Lymphatic outputs of ³H (A) and ¹⁴C (B) were expressed as percentages of the hourly dose. Lymph fistula mice were intraduodenally infused with a lipid emulsion containing [³H]TG and [¹⁴C]cholesterol for 6 h. *P < 0.05. Values are means ± SE.

Fig. 3.

Distribution of ³H (A) and ¹⁴C (B) along the small intestine divided into 4 segments of equal length, from proximal to distal: M1 (duodenum), M2 and M3 (jejunum), and M4 (ileum). *P < 0.05. Values are means ± SE.

Fig. 4.

Total recovery of ³H (A) and ¹⁴C (B) at the end of the 6-h lipid infusion. Lymph refers to the total radiolabeled lipid collected from the lymph from the 6-h infusion period. Mucosa refers to radioactivity recovered after homogenization of the small intestine. Lumen refers to luminal contents from the small intestine that were washed and collected at the end of infusion. Very low amounts of TG were recovered from the colon and stomach, indicating little excretion and reflux, respectively. *P < 0.05. Values are means ± SE.

Fig. 5.

ApoA-V KO mice have a shorter chylomicron appearance time (min) after [¹⁴C]oleic acid injection in the intestinal lumen. apoA-V KO and WT mice were infused intraduodenally with a lipid emulsion containing TG. At the 4th h of lipid infusion when steady-state lymphatic TG transport was reached, [¹⁴C]oleic acid was introduced into the lumen, and lymph samples were collected every minute afterward for 60 min. *P < 0.05. Values are means ± SE.

Fig. 6.

ApoA-V KO mice have increased intestinal apoB secretion into the lymph. A: lymph samples were analyzed for apoB by Western blot. Protein concentration was quantified by densitometric analysis and normalized to hour 0 (fasting). B: a representative blot is shown for lymphatic apoB output. C: lymph samples were analyzed for apoA-IV by Western blot.